Solid electrolyte capacitor

ABSTRACT

The invention provides a process for producing solid electrolyte capacitors by forming a dielectric oxide film and a first cathode layer of solid conductive substance over the surface of an anode body of valve metal, and forming a second cathode layer of conductive high polymer on the first cathode layer by electrolytic oxidative polymerization. The second cathode layer is formed by immersing in the electrolyte the anode body formed over the surface thereof with the oxide film and the first cathode layer, feeding a positive voltage with an external electrode piece in contact with the first cathode layer in the electrolyte, and shifting the feeding point at a predetermined time interval.

This application is a division of prior application Ser. No. 09/166,910filed Oct. 6, 1998 U.S. Pat. No. 6,168,639.

FIELD OF THE INVENTION

The present invention relates to solid electrolyte capacitors having acathode layer of electrically conductive high polymer, and to a processand an apparatus for producing such capacitors. More particularly, theinvention relates to improvements in a process for producing solidelectrolyte capacitors wherein the cathode layer is prepared from aconductive high polymer by electrolytic oxidative polymerization.

BACKGROUND OF THE INVENTION

Solid electrolytic capacitors comprise an anode body of a valve metalsuch as Al (aluminum) or Ta (tantalum), a dielectric oxide film formedon the surface of the anode body by an electrolytic oxidation treatment,and a cathode layer formed by applying an electrically conductivesubstance, such as electrolyte, MnO2 (manganese dioxide) or conductiveorganic compound, to the oxide film in intimate contact therewith. Theterm the “valve metal” as used herein refers to a metal which forms ahighly compacted durable dielectric oxide film when subjected to anelectrolytic oxidation treatment. Such metals include Ti (titanium) andNb (niobium) in addition to Al and Ta. Since the dielectric oxide filmhas a very small thickness, electrolytic capacitors have the advantagethat they can be smaller in size and greater in capacity than otherpaper capacitors and film capacitors.

Electrolytic capacitors wherein a solid conductive substance, such asMnO2 or conductive organic compound, is used for the cathode layer arecalled solid electrolyte capacitors. Examples of such conductive organiccompounds are polypyrrole, polyaniline and like conductive highpolymers, and TCNQ (7,7,8,8,-tetracyanoquinodimethane) complex salts.

These conductive organic compounds are higher than electrolytes and MnO2in electric conductivity. Accordingly, the solid electrolyte capacitorswherein the conductive organic compound is used for the cathode layerare lower in ESR (equivalent series resistance) and more excellent inhigh-frequency characteristics than when an electrolyte or MnO2 is usedfor the cathode layer. These capacitors are presently used in variouselectronic devices.

As a process for preparing the cathode layer from the conductive highpolymer among the above-mentioned conductive organic compounds, it isknown to utilize chemical oxidative polymerization or electrolyticoxidative polymerization. Chemical oxidative polymerization is a processwherein a monomer is oxidatively polymerized with use of an oxidizingagent to prepare a high polymer. Electrolytic oxidative polymerizationis a process wherein an oxidation reaction occurring on the anode inelectrolysis is utilized to subject a monomer to oxidativepolymerization and form a high polymer on the anode.

The process resorting to chemical oxidative polymerization comprisesapplying an oxidizing agent to the dielectric oxide film, and bringingthe oxidizing agent into contact with a solution or gas of the monomerto be made into a conductive high polymer to oxidatively polymerize themonomer, whereby a conductive high-polymer layer is formed on thedielectric oxide film. However, the conductive high-polymer layer formedby this process has the drawback of being low in strength, liable todevelop irregularities and lower in electric conductivity than theconductive high-polymer layer formed by electrolytic oxidativepolymerization. The process therefore fails to provide a cathode layerwhich is fully satisfactory for use in high-performance solidelectrolyte capacitors.

On the other hand, electrolytic oxidative polymerization, when resortedto, generally affords a uniform conductive high-polymer layer having ahigh strength, high electric conductivity and satisfactory quality,whereas when the conductive high-polymer layer is to be formed directlyon the dielectric oxide film by electrolytic oxidative polymerization,the oxide film, which is an insulator, fails to function as an anode,making it impossible or extremely difficult to form the high-polymerlayer on the oxide film.

Accordingly, it has been proposed to form a first cathode layer on thedielectric oxide film by a process other than electrolytic oxidativepolymerization and to subsequently effect electrolytic oxidativepolymerization with the first cathode layer serving as an anode tothereby form a second cathode layer of conductive high polymer on thefirst cathode layer.

JP-B-74853/1992 filed for Japanese patent application by Japan CarlitCo., Ltd. (U.S. Pat. No. 4,780,796 with priority claim based on thepatent application), and JP-B-65009/1991 and JP-B-23410/1992 of the samecompany, and JP-B-83167/1993 filed for Japanese patent application byNippon Chemi-Con Corp. disclose solid electrolyte capacitors wherein aconductive high-polymer layer is formed as the first cathode layer bychemical oxidative polymerization. JP-B-67767/1992 filed for Japanesepatent application by Matsushita Electric Industrial Co., Ltd. disclosesa solid electrolyte capacitor having an MnO2 layer as the first cathodelayer. Japanese Patent Application 164019/1997 not laid open and filedconjointly by Sanyo Electric Co., Ltd. and Sanyo Electronic ComponentsCo., Ltd., the assignee of the present patent application, discloses asolid electrolyte capacitor wherein a layer of TCNQ complex salt isformed as the first cathode layer.

FIG. 6 shows the common step of forming a second cathode layer from aconductive high polymer on the first cathode layer by electrolyticoxidative polymerization. An electrolyte 51 is placed in an electrolyticbath 50. The electrolyte 51 contains a monomer capable of forming aconductive high polymer, and a supporting electrolyte for giving adesired electric conductivity to the electrolyte 51. An anode body 1formed with a dielectric oxide film and a first cathode layer isimmersed in the electrolyte 51. Next, an external electrode 9 is held incontact with the first cathode layer 3 of the anode body 1, and apositive voltage is fed to the external electrode 9. The positivevoltage is fed to the first cathode layer 3 in contact with the externalelectrode 9, causing an oxidation reaction, whereby the monomer isoxidatively polymerized into a conductive high polymer. Thus, the secondcathode layer of conductive high polymer is formed on the first cathodelayer 3.

In producing solid electrolyte capacitors actually, a multiplicity ofanode bodies 1 are immersed in the electrolyte 51 as one lot andsubjected to electrolytic oxidative polymerization at the same time toform the second cathode layer on each anode body. When anode bodies 1are subsequently subjected to electrolytic oxidative polymerization asanother lot, it has been found that the electrolytic capacitors of thesubsequent lot are higher in ESR and lower in high-frequencycharacteristics than those of the previous lot. For this reason, it isconventional practice to replace the electrolyte by a fresh one everytime the polymerization operation is conducted for one lot of anodebodies 1. This entails an impaired operation efficiency and an increasedcost.

Further when a current is fed through the external electrode 9 incontact with the first cathode layer 3 for electrolytic oxidativepolymerization, the current density fails to remain constant dependingon the degree of contact between the electrode 9 and the first cathodelayer 3, presenting difficulty in forming a uniform second cathodelayer. Moreover, when the external electrode 9 is removed after thesecond cathode layer has been formed, the second cathode layer becomesdislodged locally, causing damage to the dielectric oxide film at thesame time and creating problems such as increased leakage current fromthe capacitor.

To avoid such problems, JP-A-283289/1993 filed for Japanese patentapplication conjointly by Elna Co., Ltd. and Asahi Glass Co., Ltd.discloses an external electrode 9 having a curved end 91 for use infeeding by contact with the first cathode layer 3. The electrode thusdesigned precludes a great mechanical stress from acting locally on theanode body 1, obviating the problem that the second cathode layer ispartly separated off by the removal of the external electrode 9 oncompletion of polymerization.

The electrode thus designed nevertheless has a problem: since thecurrent is fed to the first cathode layer 3 concentrically at a point P,there occurs a potential difference between the region in the vicinityof the feeding point P and a region away from this point, resulting inuneven growth of the second cathode layer in accordance with thepotential distribution.

An object of the present invention is to provide a process for producingelectrolytic capacitors wherein the electrolyte is repeatedly usable forelectrolytic oxidative polymerization to form second cathode layershaving a constant electric conductivity free of impairment.

We have found that when electrolytes are checked for pH before and afterelectrolytic oxidative polymerization, the polymerization greatly variesthe pH. This variation is thought attributable to the phenomenon thatthe conductive high polymer produced is doped with a portion or thewhole of the supporting electrolyte. Since the electric conductivity ofconductive high polymers are dependent generally on the pH of thepolymerization system, we have contrived the means to be described belowto accomplish the object.

A second object of the invention useful for producing solid electrolytecapacitors is to improve the step of forming the second cathode layer byelectrolytic oxidative polymerization with current fed to the firstcathode layer and to provide a process and an apparatus for forming thesecond cathode layer with a thickness made uniform to the greatestpossible extent.

SUMMARY OF THE INVENTION

To fulfill the foregoing objects, the present invention provides aprocess for producing a solid electrolyte capacitor including the stepsof forming a dielectric oxide film on a surface of an anode body made ofa valve metal, forming a first cathode layer of a solid electricallyconductive substance on the oxide film, and forming a second cathodelayer of an electrically conductive high polymer on the first cathodelayer by electrolytic oxidative polymerization. The step of forming thesecond cathode layer includes the step of maintaining the pH of anelectrolyte for use in the electrolytic oxidative polymerization withina predetermined range by adding an acid or alkali to the electrolyte.

Since the pH of the electrolyte remains substantially unaltered by theelectrolytic oxidative polymerization in this process, the electrolyteonce used for the polymerization is subsequently usable for thepolymerization while permitting the resulting second cathode layer tohave the desired electric conductivity. Accordingly, the sameelectrolyte is repeatedly usable, consequently achieving an improvedoperation efficiency and suppressing an increase in production cost.

The present invention also provides a solid electrolyte capacitorcomprising an anode body made of a valve metal, a dielectric oxide filmformed on a surf ace of the anode body, a first cathode layer made of anelectrically conductive high polymer formed on the oxide film bychemical oxidative polymerization, and a second cathode layer made of anelectrically conductive high polymer formed on the first cathode layerby electrolytic oxidative polymerization. The first and second cathodelayers contain the same dopant.

With the solid electrolyte capacitor of the present invention, the firstand second cathode layers contain the same dopant, which improves theconductivity between the two cathode layers, consequently givingimproved ESR characteristics to the capacitor.

The invention also provides a process for producing a solid electrolytecapacitor comprising the steps of immersing in an electrolyte an anodebody made of a valve metal and having a surface covered with adielectric oxide film and a first cathode layer of solid electricallyconductive substance, feeding a positive voltage with an externalelectrode piece in contact with the first cathode layer over the anodebody in the electrolyte to form a second cathode layer of electricallyconductive high polymer on the first cathode layer by electrolyticoxidative polymerization, and shifting a feeding point of the externalelectrode piece on the first cathode layer at a predetermined timeinterval to make the second cathode layer uniform in thickness.

The invention further provides an apparatus for subjecting toelectrolytic oxidative polymerization an anode body made of a valvemetal and having a surface covered with a dielectric oxide film and afirst cathode layer of solid electrically conductive substance to form asecond cathode layer of electrically conductive high polymer on thefirst cathode layer. The apparatus comprises an electrolytic bath forcontaining an electrolyte, means for supporting the anode body asimmersed in the electrolyte in the bath, a plurality of externalelectrode pieces arranged in the bath and so supported as to be movableinto and out of contact with the anode body, and a change-over devicecoupled to the external electrode pieces for changing over feeding fromone of the electrode pieces to the other electrode piece. A feedingpoint of the electrode piece on the first cathode layer is repeatedlyshift by the change-over device.

With the process and apparatus described above for producing solidelectrolyte capacitors, the feeding point where the electrode piece isin contact with the first cathode layer for feeding shifts during thestep of electrolytic oxidative polymerization, so that the thickness ofthe second cathode layer formed as centered about the feeding point ismade uniform to the greatest possible extent over the entire anode bodywithout increasing locally.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing the structure of a known capacitorelement which is the main portion of a solid electrolyte capacitor;

FIG. 2 is a front view showing a production apparatus as a firstembodiment;

FIG. 3 includes front views showing a production apparatus according toa second embodiment, (A) and (B) being views showing a feeding point asshifted;

FIG. 4 includes front views showing another production apparatusaccording to the second embodiment, (A) and (B) being views showing afeeding point as shifted;

FIG. 5 is a front view showing another production apparatus according tothe second embodiment; and

FIG. 6 is a front view of a conventional production apparatus.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described below in greater detail.

EMBODIMENT 1

FIG. 1 shows a known capacitor element 100 which is the main portion ofa solid electrolyte capacitor and which comprises an anode body 1 madeof a valve mental such as Al or Ta. An anode lead wire 11 is welded oradhered to the anode body 1, and the anode body 1 and a portion of thelead wire 11 are treated by electrolytic oxidation over the surfacesthereof to form a dielectric oxide film 2. The oxide film 2 is formedwith a first cathode layer 3, on which a second cathode layer 4 ofelectrically conductive high polymer is formed by electrolytic oxidativepolymerization. A carbon layer and a silver paste layer are formed overthe second cathode layer 4 of the capacitor element 100 thus prepared, ametal terminal plate is attached to each of the anode lead wire 11 andthe silver paste layer, and the assembly is encapsulated with an epoxyresin or the like, followed by aging, to complete a solid electrolytecapacitor.

The anode body 1 is the form of a metal foil or sintered metal body. Themetal foil is rough-surfaced by etching and thereby given an increasedsurface area. The sintered metal body is prepared by sintering aparticulate metal and is porous. In the case where the sintered metalbody is used as the anode body 1, the dielectric oxide film 2 and thecathode layers 3, 4 are formed over the outer surface thereof and alsoinside the pores. Sintered bodies 10 of Ta serving as a valve metal areused in Example and Comparative Example to be given below.

Electrically conductive organic compounds such as conductive highpolymers and TCNQ complex salts, or electrically conductive inorganiccompounds such as manganese dioxide are usable for forming the firstcathode layer 3. Polypyrrole which is a conductive high polymer is usedfor forming the layer 3 and prepared by chemical oxidativepolymerization in Example and Comparative Example to be given later.

Polypyrrole, polyaniline, polythiophene, polyfuran and derivatives ofthese polymers are usable for forming the second cathode layer 4 ofconductive high polymer. Polypyrrole is used in Example and ComparativeExample to be given later.

Conductive high polymers exhibit metallic properties and are remarkablyincreased in electric conductivity when doped with a suitable substancein the interior. In the case of solid electrolyte capacitors whereinconductive high polymers are used for the cathode layer, the conductivehigh polymer is generally doped with a suitable substance. The substanceto be used for doping is termed a “dopant.”

To form the first cathode layer 3 by chemical oxidative polymerization,the anode body 1 formed with the dielectric oxide film 2 is first dippedin a solution containing an oxidizing agent and a doping agent forgiving the dopant, or sprayed or coated with the solution, whereby theoxidizing agent and the doping agent are applied to the oxide film 2 onthe anode body.

Usable as the oxidizing agent is any of those generally known, such ashalogens and peroxides.

Examples of useful doping agents are protonic acids including sulfuricacid and nitric acid, and surfactants such as alkylsulfonic acid salts.Examples of other compounds useful as doping agents are described indetail in JP-B-83167/1993 mentioned above, and other literature (e.g.,K. Yoshino and M. Onoda, “Polymer Electronics, ” Corona Publishing Co.,Ltd., 1996).

An element or compound (such as halogen, transition metal halide orprotonic acid) which is serviceable as an oxidizing agent and also as adoping agent may be made into a solution, and the solution need notcontain two kinds of compounds serving as oxidizing agent and dopingagent, respectively.

Next, a solution or gas of a monomer capable of forming a conductivehigh polymer is then brought into contact with the anode body 1 thustreated, by dipping, spraying or coating. The monomer is thenoxidatively polymerized by the oxidizing agent, forming a cathode layer3 of conductive high polymer on the dielectric oxide film 2.

FIG. 2 shows a production apparatus 5 by which the second cathode layer4 of conductive high polymer is formed by electrolytic oxidativepolymerization on the first cathode layer 3 of the anode body 1resulting from the above treatment. The apparatus 5 comprises anelectrolytic bath 50 containing an electrolyte 51.

The electrolyte 51 contains a monomer capable of forming the conductivehigh polymer, a supporting electrolyte and other additives. Thesupporting electrolyte is strongly electrolytic, added to give a desiredelectric conductivity to the electrolyte 51 and suitably selected inconformity with the monomer and solvent to be used. It is especiallydesired that the supporting electrolyte be a doping agent serving as thedopant for the conductive high polymer. Used in Example and ComparativeExample to be given below are pyrrole insoluble in water and serving asthe monomer, water as the solvent, and a sodiumalkylnaphthalenesulfonate which is a surfactant and stronglyelectrolytic and serves as the supporting electrolyte.

Disposed above the electrolyte 51 is a carrier bar 52 for holding amultiplicity of anode bodies 1 treated as stated above and immersing theanode bodies 1 in the electrolyte 51. The anode lead wires 11 of theanode bodies 1 are attached to the carrier bar 52 by welding. Theproduction apparatus 5 generally has many carrier bars 52, and all theanode bodies 1 attached to these carrier bars 52 are subjected toelectrolytic oxidative polymerization at the same time as one lot.

Arranged in the electrolyte 51 are a pair of external electrodes 53, 54for electrolysis. The electrode 53 of the pair is connected to apositive electrode of a power source 55 and positioned so as to contactthe first cathode layer 3 of each anode body 1. The other externalelectrode 54 is connected to a negative electrode of the power source55. In the following description, the external electrode 53 will betermed the “anode electrode,” and the other external electrode 54 the“cathode electrode.”

According to the present embodiment, a commercial pH meter 56 isprovided for measuring the pH of the electrolyte 51. The pH meter 56comprises an electrode portion 56 a immersed in the electrolyte 51, anda body portion 56 b for calculating the pH of the electrolyte 51. Usedas the pH meter 56 is Model M-13, a product of Horiba, Ltd.

Further with the present embodiment, there is disposed a supply tank 57containing an acid or alkali solution for maintaining the electrolyte 51at the desired pH. In accordance with the pH value measured by the pHmeter 56, the acid or alkali solution is suitably supplied from the tank57 to the electrolytic bath 51. The acid or alkali solution to be usedis preferably a doping agent like the supporting electrolyte describedabove although not limited specifically. The supply tank 57 containssulfuric acid in Example to follow.

When the apparatus 5 thus constructed is operated with a current fedfrom the power source 55 to the anode electrode 53 and the cathodeelectrode 54, electrolysis takes place, causing the monomer to undergoan oxidative polymerization reaction on the anode electrode 53 and thefirst cathode layer 3 in contact with this electrode 53, whereby asecond cathode layer 4 of conductive high polymer is formed.

The present invention will be described below with reference to thefollowing example and comparative example.

a. EXAMPLE

First, sintered bodies 10 of Ta each having an anode lead wire 11attached thereto were immersed in an aqueous solution of phosphoric acid(0.02 wt % (weight percentage)), and a voltage was impressed on thesintered bodies for electrolytic oxidation to form a dielectric oxidefilm 2 on the outer surface of each body 10, inside the pores thereofand on the surface of a portion of the lead wire 11.

For chemical oxidative polymerization, an aqueous solution was thenprepared which contained hydrogen peroxide (1 mole/liter inconcentration) serving as an oxidizing agent and sulfuric acid (0.2mole/liter in concentration) as a doping agent. The sintered Ta body 10treated as above was immersed in the aqueous solution for 10 minutes andthereafter exposed to pyrrole monomer for 30 minutes to effect chemicaloxidative polymerization, whereby a first cathode layer 3 of polypyrrolewas formed on the surface of the dielectric oxide film 2.

For electrolytic oxidative polymerization, an electrolyte 51 wassubsequently prepared which contained pyrrole monomer and a supportingelectrolyte, i.e., a sodium alkylnaphthalenesulfonate, in water servingas a solvent, and which was adjusted to a pH of 3 with sulfuric acid. Asshown in FIG. 2, each sintered Ta body 10 treated as described above wasimmersed in the electrolyte 51, and a current was fed across the anodeelectrode 53 and the cathode electrode 54, with the anode electrode 53in contact with the first cathode layer 3 to effect electrolyticoxidative polymerization, whereby a second cathode layer 4 ofpolypyrrole was formed on the surface of the first cathode layer 3.During the feeding, the electrolyte 51 was checked for pH by the pHmeter 56, and sulfuric acid was suitably added to the bath 50 from thesupply tank 57 so as to maintain the electrolyte 51 at a pH of up to 8.

The sintered body was thereafter washed and dried to complete acapacitor element 100, followed by the same steps as previouslydescribed to complete a solid electrolyte capacitor.

The measurements obtained in Example were as follows.

The electrolyte 51 as prepared before the electrolytic oxidativepolymerization step had an alkylnaphthalenesulfonate ion concentrationof 0.06 mole/liter. After performing the electrolytic oxidativepolymerization step five times, the electrolyte 51 was 0.051 mole/literin the sulfonate ion concentration. Thus, the polymerization resulted inonly a slight decrease in the ion concentration.

When the completed solid electrolyte capacitors were checked for ESR,the average value was 47 milliohms, the maximum was 51 milliohms and theminimum was 43 milliohms.

b. COMPARATIVE EXAMPLE

In this comparative example, solid electrolyte capacitors were completedin the same manner as in Example except that no sulfuric acid was addedto the bath in the electrolytic oxidative polymerization step. Thisproduction process is the same as the conventional process.

Stated more specifically, prepared for electrolytic oxidativepolymerization was an electrolyte 51 containing pyrrole monomer and asupporting electrolyte, i.e., a sodium alkylnaphthalenesulfonate, inwater serving as a solvent. Sintered Ta bodies 10 treated as describedabove was immersed in the electrolyte 51, and a current was fed acrossthe anode electrode 53 and the cathode electrode 54, with the anodeelectrode 53 in contact with the first cathode layer 3 of each sinteredbody 10 to effect electrolytic oxidative polymerization, whereby asecond cathode layer 4 of polypyrrole was formed on the surface of thefirst cathode layer 3.

Comparative Example gave the following measurements.

The electrolyte 51 as prepared before the electrolytic oxidativepolymerization step had an alkylnaphthalenesulfonate ion concentrationof 0.06 mole/liter, which was the same value as in Example. The pH ofthe electrolyte 51 was about 7. After performing the electrolyticoxidative polymerization step once, the electrolyte 51 was 0.052mole/liter in the sulfonate ion concentration. The decrease in the ionconcentration was comparable to that resulting from the polymerizationstep performed five times in Example. The electrolytic oxidativepolymerization step performed once gave a pH of about 10 to theelectrolyte 51. This appears to indicate that the polypyrrole formed bythe electrolytic oxidative polymerization was doped withalkylnaphthalenesulfonate ions, with the electrolyte 51 made alkalinewith the remaining sodium ions.

When the completed solid electrolyte capacitors were checked for ESR,the average value was 61 milliohms, the maximum was 80 milliohms and theminimum was 50 milliohms.

c. DISCUSSION Comparison between Example of the invention andComparative Example as to the measurements obtained indicates thatExample showed the following effects.

The polypyrrole polymerized in an alkaline solution is generally lowerin electric conductivity than that polymerized in an acid or neutralsolution. The electrolyte used in Comparative Example for performing theelectrolytic oxidative polymerization step only once becomes alkalineand therefore needs to be replaced before producing the subsequent lotof capacitor elements. In Example of the invention, on the other hand,the electrolyte remains acid or neutral even after it is used forperforming the polymerization step several times, so that theelectrolyte is usable repeatedly. This achieves an improved operationefficiency and a reduced cost in manufacturing capacitors.

Further a comparison between Example and Comparative Example as to thedifference in ESR between the maximum and the minimum reveals thatExample affords capacitors with more stabilized ESR characteristics thanComparative Example. Presumably this is attributable to the variationsin the pH of the electrolytic oxidative polymerization system ofComparative Example, in contrast with Example wherein the electrolytecan be maintained at an optimum pH value.

Further according to Example, the electrolytic oxidative polymerizationconducted reduces the alkylnaphthalenesulfonate ion concentration onlyslightly. This appears to indicate that the polypyrrole formed by thepolymerization in Example does not associate with the supportingelectrolyte but is doped mainly with sulfate ions. Accordinglyinexpensive sulfuric acid is usable as the acid to be added to theelectrolytic bath 51, and there is no need to add an acid containing thesame anion as the supporting electrolyte (i.e., analkylnaphthalenesulfonic acid in Example).

When further compared with Comparative Example in the average value ofESR, Example provides capacitors of more excellent ESR characteristicsthan Comparative Example. Since the polypyrrole formed by chemicaloxidative polymerization and that formed by electrolytic oxidativepolymerization are doped with the same agent, i.e., sulfate ion, it isthought that the same dopant serving for both the first cathode layer 3and the second cathode layer 4 improved the cathode layers in electricconductivity.

Because the amount of the supporting electroltye doping the polypyrroleis very small, an electrolyte inappropriate as a dopant for thepolypyrrole can be used as a supporting electrolyte.

For example, sodium alkylbenzenesulfonates are currently less expensivethan sodium alkylnaphthalenesulfonates. However, the polypyrrole dopedwith the sodium alkylbenzenesulfonate has lower heat resistance than thepolypyrrole doped with the sodium alkylnaphthalenesulfonate. For thisreason, the sodium alkylnaphthalnesulfonate is solely used in the priorart as a supporting electrolyte rather than the sodiumalkylbenzenesulfonate. According to Example, the sodiumalkylbenzenesulfonate is usable as a supporting electrolyte in place ofthe sodium alkylnaphthalenesuflonate without resulting in impaired heatresistance since the polypyrrole is doped mainly with sulfate ions.

Polypyrrole is used for forming the cathode layers according to Example,whereas if polyaniline, for example, is to be used for the cathodelayers, there a rises a need to maintain the electrolyte at an aciditypH value of less than 7 in the electrolytic oxidative polymerizationstep since polyaniline is conductive only when polymerized in an acidsolution. Thus the pH range in which the electrolyte must be maintainedneeds to be altered depending on the conductive high polymer to beformed and the dopant.

EMBODIMENT 2

This embodiment relates to the step of forming by electrolytic oxidativepolymerization the second cathode layer 4 as uniformly as possible onthe first cathode layer 3. The treatment steps other than thiselectrolytic oxidative polymerization step are the same as in the firstembodiment described.

FIGS. 3 to 5 show the main arrangement included in a productionapparatus for forming the second cathode layer 4. This apparatus has thesame construction as the conventional one shown in FIG. 6 except theexternal electrode (anode electrode) to be in contact with the anodebody 1 and a device related to the electrode. The anode body 1 having adielectric oxide film and a first cathode layer 3 over the surface ofthe body is supported in an electrolytic bath 50 and immersed in anelectrolyte 51 which is an acetonitrile solution of tetraethylammoniump-toluenesulfonic acid (0.05 mole/liter in concentration) and pyrrolemonomer (0.1 mole/liter in concentration).

As shown in FIG. 3, the electrolytic bath 50 is provided with an anodeelectrode piece 6 which is movable into and out of contact with theanode body 1. The anode electrode piece 6 is connected to the positiveelectrode of a power source 55. As seen in FIG. 5, a cathode electrode54 connected to the negative electrode of the power source 55 isdisposed in the electrolyte 51.

The region of the first cathode layer 3 where the anode electrode piece6 comes into contact with the layer 3 serves as a feeding point P,through which a current is fed to the first cathode layer 3 to form asecond cathode layer 4 of polypyrrole on the first cathode layer 3 byelectrolytic oxidative polymerization.

The anode electrode piece 6 is in the form of a resilient metal piece.Instead of vertical contact as shown in FIG. 3, the electrode piece 6 ispreferably pressed sideways into contact with the anode body 1 undersmall pressure as shown in FIG. 4 by being elastically deformed. Thisensures stabilized feeding to the first cathode layer 3 and diminishesmechanical impact on the layer 3, precluding damage to the first cathodelayer 3 and the base layer therefor, i.e., the dielectric oxide film 2.

The anode electrode piece 6 is supported by a change-over device 7 onthe electrolytic bath 50 so as to change the feeding point P on theanode body 1 within the bath 50. A known horizontal moving mechanism,pivotal mechanism, lift mechanism or any other mechanism is usable asthe change-over device 7 for making the anode electrode piece 6 movableinto and out of contact with the anode body 1. FIGS. 3 to 5 show some ofsuch mechanisms.

FIG. 3 shows a pair of anode electrode pieces 6, 6 a arranged atopposite sides of the anode body 1 and supported by the change-overdevice 7 so as to be horizontally movable at the same time toward oraway from opposite sides of the anode body 1. When the electrode piece 6is in contact with the left side of the anode body 1 for feeding, thesecond cathode layer 4 is formed, as centered about the feeding point P,to a greater thickness at the left side of the anode body 1 than at theright side thereof. When the change-over device 7 subsequentlyfunctions, the other electrode piece 6 a comes into contact with theright side of the anode body 1 as seen in FIG. 3B, shifting the feedingpoint P to the right side of the anode body 1 to form the conductivehigh-polymer layer 4 to a greater thickness on the right side of theanode body 1. The change-over device 7 is actuated at a suitable timeinterval, e.g., at an interval of about 30 minutes to shift the feedingpoint P to opposite sides alternately, whereby the second cathode layer4 is given a uniform thickness on the left and right sides of the anodebody 1.

With reference to FIG. 4, the anode electrode piece 6 is upwardly anddownwardly movably supported by a change-over device 7 to come intocontact with a low portion and a high portion of the anode body 1alternately to form a uniform cathode layer 4 over the entire length ofthe anode body 1. The feeding point P is made shiftable to the left andright sides of the anode body 1 and to upper and lower portions thereofby the embodiments of FIGS. 3 and 4 in combination.

With reference to FIG. 5, a pair of anode electrode pieces 6, 6 a arearranged at opposite sides of the anode body 1 symmetrically and broughtinto contact with the anode body 1 at the same time, and feedingcircuits connected to the power source 55 and provided for therespective electrode pieces are alternately changed over by achange-over device 7.

With the embodiment of FIGS. 3 and 5, the electrode pieces 6, 6 a arerepeatedly brought into and out of contact with the anode body 1,interfering with the formation of the second cathode layer 4 by contactat the feeding point P, while when moving out of contact with the anodebody, the external electrode piece 6 locally separates the secondcathode layer 4 from the surface of the first cathode layer 3. In thesubsequent cycle, however, the electrode piece 6 moves, freeing theregion of the previous feeding point on the first cathode layer 3 tofreshly form or repair the second cathode layer. This also gives auniform thickness to the second cathode layer 4, giving improvedhigh-frequency characteristics to the solid electrolyte capacitors to beproduced.

When the second cathode layer 4 is formed to a predetermined thickness,the anode electrode piece 6 is moved away from the anode body 1, theanode body 1 is taken out of the bath 50, washed and dried to complete acapacitor element 100. Solid electrolyte capacitor is thereaftercompleted by the same steps as previously described.

The foregoing description of embodiments is intended to illustrate thepresent invention and should not be construed as limiting the inventionset forth in the appended claims or reducing the scope thereof. Theprocess and apparatus of the invention are not limited to theembodiments but can of course be modified variously within the technicalscope defined in the claims.

What is claimed is:
 1. A solid electrolyte capacitor comprising: ananode body made of a valve metal, a dielectric oxide film formed on asurface of the anode body, a first cathode layer made of an electricallyconductive high polymer formed on the oxide film by chemical oxidativepolymerization, the first cathode layer containing a dopant, and asecond cathode layer made of an electrically conductive high polymerformed on the first cathode layer by electrolytic oxidativepolymerization, the second cathode layer containing the same dopant asthe first cathode layer.
 2. A solid electrolyte capacitor according toclaim 1 wherein the dopant is sulfate ion.
 3. A solid electrolytecapacitor according to claim 2 wherein the conductive high polymers ofthe first and second cathode layers are both polypyrrole.
 4. A solidelectrolyte capacitor comprising: an anode body made of a valve metal, adielectric oxide film formed on a surface of the anode body, a firstcathode layer formed on the dielectric oxide film, and a second cathodelayer formed on the first cathode layer, wherein the second cathodelayer consists of a conductive high polymer which is doped with analkylnaphthalenesulfonate ion and a sulfate ion.
 5. A solid electrolytecapacitor according to claim 4 wherein the first cathode layer consistsof a conductive high polymer which is doped with a sulfate ion.
 6. Asolid electrolyte capacitor according to claim 4 wherein the conductivehigh polymer of the second cathode layer is polypyrrole.
 7. A solidelectrolyte capacitor comprising: an anode body made of a valve metal, adielectric oxide film formed on a surface of the anode body, a firstcathode layer formed on the dielectric oxide film, and a second cathodelayer formed on the first cathode layer, wherein the second cathodelayer consists of a conductive high polymer which is doped with analkylbenzenesulfonate ion and a sulfate ion.
 8. A solid electrolytecapacitor according to claim 7 wherein the first cathode layer consistsof the conductive high polymer which is doped with a sulfate ion.